The first man-made plastic was invented in Britain in 1851 by Alexander PARKES. He publicly demonstrated it at the 1862 International Exhibition in London, calling the material Parkesine. Derived from cellulose, Parkesine could be heated, molded, and retain its shape when cooled. It was, however, expensive to produce, prone to cracking, and highly flammable. In 1868, American inventor John Wesley HYATT developed a plastic material he named Celluloid, improving on PARKES' invention so that it could be processed into finished form. HYATT patented the first injection molding machine in 1872. It worked like a large hypodermic needle, using a plunger to inject plastic through a heated cylinder into a mold. The industry expanded rapidly in the 1940s because World War II created a huge demand for inexpensive, mass-produced products. In 1946, American inventor James Watson HENDRY built the first screw injection machine. This machine also allowed material to be mixed before injection, so that colored or recycled plastic could be added to virgin material and mixed thoroughly before being injected. In the 1970s, HENDRY went on to develop the first gas-assisted injection molding process.
Injection molding machines consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. They are also known as presses, they hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection process. Tonnage can vary from less than five tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations. The total clamp force needed is determined by the projected area of the part being molded. This projected area is multiplied by a clamp force of from two to eight tons for each square inch of the projected areas. As a rule of thumb, four or five tons per square inch can be used for most products.
If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage to hold the mold closed. The required force can also be determined by the material used and the size of the part, larger parts require higher clamping force. With Injection Molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heated chamber, where it is melted. As the plunger advances, the melted splastic is forced through a nozzle that rests against the mold, allowing it to enter the mold cavity through a gate and runner system. The mold remains cold so the plastic solidifies almost as soon as the mold is filled. Mold assembly or die are terms used to describe the tooling used to produce plastic parts in molding. The mold assembly is used in mass production where thousands of parts are produced. Molds are typically constructed from hardened steel, etc. Hot-runner systems are used in molding systems, along with mold assemblies, for the manufacture of plastic articles. Usually, hot-runners systems and mold assemblies are treated as tools that may be sold and supplied separately from molding systems.
U.S. Pat. No. 6,540,496 (Inventor: SCHAD, et al.; Filed: 30 Jun. 1999) discloses an injection molding machine having shooting pots for charging an injectable material into the mold cavity, each said shooting pot having an injection plunger for expressing said material from said shooting pot into said mold cavity, a shooting pot actuator including a pusher for each shooting pot, a linear position sensor operably attached to a control system, for sensing each said shooting pot actuator; and a drive means responsive to said control system and operable to separately move the pushers between said retracted and second positions. Specifically, FIG. 3 depicts the following: the position and linear velocity of plates 222 and 224 can be sensed by linear position sensor means 240. Sensor 240 can be a magnetic, opto-electronic or other suitable sensor, such as those manufactured by Temposonic Inc. Sensor 240 is fixed to frame 198, or otherwise fixed relative to plates 222 and 224. The sensor 240 can be attached to a suitable control system (not shown) for conventional electronic and/or programmable control of the actuator 200, as is well known to those of skill in the art. The position and speed of plate 224 during the forward stroke is sensed by sensor 240. Sensor 240 relays the information to the control system which in turn, controls the speed and distance traveled by the pushers 228. The position and speed of plate 222 are sensed by sensor 240 to control the speed and distance travelled by the pushers 226, as described above.
U.S. Pat. No. 6,491,509 (Inventor: SCHAD, et al.; Filed: 10 Dec. 2002) discloses an injection molding machine that has common control of multiple shooting pots in an injection molding machine, and that individual control of the shooting pot strokes is provided in these prior art injection molding machines. Separate hydraulic actuation cylinders for each shooting pot injection plunger are mounted inside the machine's stationary platen. These hydraulic cylinders must be individually set for stroke to control the individual metering of the resins into the mold cavities.
United States Patent Publication Number 20090274790 (JENKO, et al.; Filed: 17 Jul. 2009) discloses a hot runner system including a shoot pot system for transferring melt from a single shooting pot to multiple nozzles. Melt is fed from a source of melt into the cavity through the multiple nozzles, and a valve isolates melt in the cavity from melt in the source. A plunger within the cavity is driven forward to inject melt in the cavity into a mold cavity at high pressure without significantly increasing the pressure of melt in the source. The plunger optionally functions as both the plunger and the valve by opening and closing communication between the cavity and the manifold as it is rotated. Specifically, synchronized filling and consistent part weight can be adjusted and controlled through plunger 34 start and stop positions. These start and stop positions can be confirmed with sensors for greater precision and reliability.